In recent years, with a view point of environmental problems and energy crises, there are increasing expectations for hybrid vehicles and electric vehicles. With such background, there is a demand for an energy storage device that can be repeatedly charged and discharged and provide high energy.
For example, JP-H08-124568A corresponding to U.S. Pat. No. 5,795,679B discloses an electric power storage device such as a lithium secondary battery having a high energy density and a long cycle life. Specifically, from an alloy powder of an amphoteric metal that forms a negative electrode, part of amphoteric metal is selectively eluted and etched. That is, a technology for obtaining a negative electrode material which is uniform and has a large specific surface area is disclosed.
However, while the lithium ion secondary battery is a prospective electric power storage device, the storable energy density is limited.
Then, in order to develop an electric power storage device that can provide more excellent energy density than the lithium ion secondary battery, the inventors of the present application have studied a magnesium secondary battery as such a device. Discussion based on the inventors' studies will be given below as related arts.
Magnesium, which is generally used as a negative electrode material in a magnesium secondary battery, is an extremely oxidizable metal. According to the diagram of free energy of oxide formation and temperature and oxygen partial pressure (so-called Ellingham diagram), an equilibrium oxygen partial pressure of magnesium and magnesium oxide at a room temperature is about 10−200 Pa. This means that the oxidation reaction of magnesium proceeds at an oxygen partial pressure equal to or higher than the above-described level. There is a problem that, as well as in atmospheric air, an oxide is formed at a surface of magnesium even in a globe box filled with an inert gas used upon manufacture of a battery and samples for analysis.
In the magnesium secondary battery, elemental magnesium is often used as a negative electrode active material. The electric resistivity of a magnesium oxide mainly formed from elemental magnesium is outstandingly higher than that of elemental magnesium (compound containing a magnesium oxide that increases the impedance of an electrode is hereinafter referred to as “high impedance compound”).
Actually, the inventors of the present application assembled magnesium secondary batteries in a globe box by substantially the same method as in the later-described embodiment and measured the impedance of the magnesium negative electrode by substantially the same method as in the later-described embodiment. As illustrated in FIG. 20, the impedance of the magnesium negative electrode varied from several thousands Ωcm2 to several tens thousands Ωcm2. When three electrode cells were assembled in the globe box, the surface of the magnesium negative electrode was polished by a glass edge, to remove the magnesium oxide formed on the surface of the elemental magnesium. At a moment when the instance elemental magnesium was polished, a pure metal surface of magnesium was exposed. However, magnesium at the pure metal, surface reacts with oxygen remaining in the globe box. As a result, the impedance of the magnesium negative electrode was increased even just after the assembling of the three electrode cell. On the other hand, when substantially the same experiment was performed for elemental lithium, the impedance of the lithium negative electrode was stable at several tens Ωcm2.
High impedance of the electrode means that the overvoltage of the electrode increases when the current density is high. For example, the negative electrode impedance of the magnesium secondary battery is assumed to be 10,000 Ωcm2. When the battery is charged/discharged at a current density of 1 mA/cm2, the overvoltage of the negative electrode is 10 V. Generally, the electromotive force of a secondary battery is several V. Accordingly, there is a relation: electromotive force<negative electrode overvoltage. Charging/discharging at a current density of 1 mA/cm2 is impossible.